Interpretive Summary: Poultry coccidiosis is caused by several different species of a protozoan intestinal parasite which causes considerable annual losses to the poultry industry. The primary control for the disease is through applied medications in the feed as birds are raised in confinement in broiler houses. The control by medications is becoming less effective because of increased resistance to the drugs and less desirable due to concerns about the possibility of drugs remaining in the meat and within the environment. As an alternative to drugs, immunization with small numbers of oocysts (the infective stage of the parasite) is currently used to induce immunity in birds and to protect them from disease. The use of live parasites in vaccines has some drawbacks, including introduction of pathogens into the environment (which produce large number of oocysts thus adding additional parasites to a broiler house). As an alternative, we used gamma irradiation of oocysts to attenuate the development of the parasite, thus reducing pathology and fecundity. Chicks, raised in conditions similar to those in broiler houses, were inoculated with irradiated oocysts from three major species of coccidia. The results of this experiment determined the optimum dose of radiation to attenuate the development of the parasite and reduce the output of oocysts while protecting birds from infection. Further studies done using this optimum dose of irradiation indicated that immunization of chicks, with small numbers of oocysts, conveyed protection to infection by three species of coccidia in a manner similar to that observed using virulent, non-irradiated parasites. Thus, irradiated oocysts present a viable alternative to virulent oocysts for use in a coccidiosis vaccine.

Technical Abstract:
The current study investigates the use of irradiated oocysts to protect broiler chicks, raised on litter, from infection with multiple species of Eimeria. In order to determine the optimum radiation dose for each Eimeria species, day- old chicks were immunized with oocysts of E. maxima, E. acervulina or E. tenella exposed to gamma radiation ranging from 5 to 500 Gy. The litter oocyst counts at 7 days post- immunization and the effect on weight gain following a challenge infection decreased with an optimum dose between150 - 200 Gy. Based on this finding broiler chicks were immunized with a mixture of E. maxima, E. acervulina and E. tenella that were exposed to 150 or 200 Gy. Immunization of chicks with oocysts from E. maxima, E. acervulina and E. tenella irradiated with 150 Gy resulted in more than a 100 fold reduction in litter oocyst counts and significant protection from a challenge infection as measured by improved weight gain and feed conversion ratio (FCR). Immunization of birds with oocysts receiving 200 Gy was less effective in providing protection from a challenge infection. An additional formulation of vaccines containing two different oocyst doses of the 3 species that were irradiated with 150 Gy were evaluated in their ability to attenuate oocyst output and convey protection to challenge. Results were similar with both high and low numbers of irradiated oocysts. Immunized chicks produced lower numbers of oocysts at 7 days post- immunization and were protected from negative effects of challenge infection as measured by feed conversion ratio (FCR), changes in weight gain, lesion scores and measurement of body composition. However, the level of protection was somewhat less than achieved by immunization with nonirradiated oocysts. The overall conclusion is that an irradiated oocyst vaccine developed in this study can effectively protect chicks that are raised on litter from challenge infection with multiple species of Eimeria, comparable to vaccines with virulent or precocious strains.